An HTML validator is a quality assurance program used to check Hypertext Markup Language ( HTML ) markup elements for syntax errors. A validator can be a useful tool for an HTML user who receives data electronically from a variety of input sources. Syntax errors, such as open tags, extra spaces, or forgotten quotation marks, can cause a Web page to look drastically different than the creator intended, or render correctly in one browser , but not in another. Verifying the integrity of HTML elements manually is repetitive and tedious work; it becomes even more complicated when different sets of validation rules, such as those for cascading style sheets ( CSS ) and Extensible Markup Language ( XML ) are included in the page. By using a validation program to systematically flag errors, the HTML user can choose whether to correct code mistakes on a case-by-case basis or correct them globally by using a "find and replace" mechanism within the program.